The production of mechanical pulps is of increasing interest since a higher yield can be obtained from a given amount of raw material utilizing mechanical pulping processes as compared to chemical pulping processes. Mechanical pulping, in general, refers to refiner mechanical pulping (RMP), thermomechanical pulping (TMP), chemimechanical pulping (CMP), and chemithermomechanical pulping (CTMP) and board pulping. In each case a refiner (defibrator) is utilized as one of the basic components for breaking down the chips into progressively smaller bundles in a fibrillation process. Despite the high yields available for mechanical pulps, in many areas of the world the production of such pulps is not economical because of the energy intensive nature of the fibrillation process. Typically the refiner is driven with an electrical motor, and where electricity costs are high it is not economically feasible to utilize mechanical pulping processes.
According to the present invention, a method and apparatus are provided which allow mechanical pulping to be economically feasible under a wider range of circumstances than is the case presently. According to the invention, advantage is taken of the fact that during the fibrillation process, water in the chips and liquid that is pumped into the refiner during the refining process evaporates as a result of the frictional heat generated in the refiner, to produce process steam which exits with the pulp. In conventional refiners, an amount, 30-50%, of the process steam usually inherently vents out of the refiner inlet. However the majority of the process steam is discharged from the refiner along with the pulp, and typically is passed to a centrifugal separator where the steam is separated from the pulp. In some refining systems, the steam which vents out of the refiner inlet is combined with the pulp and steam from the pulp outlet and the combined steam is passed to a centrifugal separator. In such systems practically all steam generated in the refiner is available as one stream after the centrifugal separator and at a pressure practically equal to that prevailing in the refiner.
The steam that is vented off the centrifugal fiber separator typically contains impurities such as volatile components of the wood. In a typical system this steam is fed to a heat exchanger (often referred to as reboiler or steam transformer) where the process steam is condensed thereby providing heat for the evaporation of boiler feed water and clean steam is produced. Typically this steam is used for drying paper or pulp and reduces the demand for steam produced in boilers burning oil, coal, wood wastes, etc.
According to the present invention, the steam produced in the reboiler is used to drive a steam turbine which is operatively connected to the drive shaft for the refiner. If desired, a portion of the process steam itself may be utilized to effect presteaming of the chips before they are fed to the refiner, and additionally the steam discharge from the turbine may be utilized to effect presteaming. Where the turbine is of the condenser type, the condensate is used as feed water to the reboiler.
A number of advantages ensue from the practice of the invention. By the practice of the invention it is possible to provide a significant portion of the energy that is necessary to run the refiner from the process steam itself. Therefore only a part of the energy typically necessary to power the refiner need be provided from an accessory source, such an electric motor, or such as from steam generated by the burning of oil, coal, bark or like waste products. Thus the production of mechanical pulps is economically feasible even in countries where electrical costs are high. Further, according to the invention it is possible to more precisely control the refiner speed so that it is optimum for the particular material being treated thereby enhancing pulp quality.
In the practice of the invention, a wide variety of structures may be utilized for adding the additional energy necessary to power the refiner. For instance a steam ejector may introduce steam under pressure from an accessory source to the turbine to supplement the steam from the reboiler. Alternatively, a mechanical steam compressor may be provided between the reboiler and the turbine. The mechanical steam compressor may be driven by an electric motor; or it may be driven by the turbine output shaft itself, and an additional supply of steam introduced between the mechanical steam compressor and the turbine. Another alternative is to provide the steam turbine as a low pressure turbine and additionally provide a high pressure turbine operatively connected to the refiner drive shaft, with fresh steam led to the high pressure turbine, and then the discharged steam from the high pressure turbine being added to the steam from the reboiler to the low pressure turbine. Still another modification is to provide an electrical motor directly connected to the refiner drive shaft.
The turbine is preferably connected to the refiner through reduction gearing means. In this way the refiner rpm can be precisely controlled for optimum conditions. Alternatively, the turbine can drive a hydraulic pump, which in turn drives a hydraulic motor connected to the refiner drive shaft. The turbine, and accessory components, can be utilized to drive not just one refiner, but rather a plurality of refiners connected in parallel or in series.
Since the process steam from the refiner has a low value, it is highly desirable to increase the efficiency of the operation by adding additional energy to the process steam. This is preferably effected by--after passage of the process steam through a reboiler--super heating the steam. Super heating may be accomplished either by passing the steam through a super heater heat exchanger associated with a boiler for the pulp mill, or passing it through an economizer associated with the off-gases of combustion from the boiler. Additional steam energy supplied to the steam turbine is provided by steam produced by the boiler, either passed directly from the boiler to the turbine, or passed through another turbine which drives an electrical generator, and then from the other turbine to the refiner driving steam turbine. Where an economizer in the combustion off-gases is utilized, the off-gases preferably are first passed through a gas turbine which drives an electrical generator and an air compressor, with the discharge from the gas turbine providing the economizer. For small movable mills, the mills can be mounted on a train and the locomotive for the train powered by the gas turbine.
It is the primary object of the present invention to provide a method and apparatus for the economical production of mechanical pulps. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.